A fixed bed gasifier commonly comprises a vertical water jacketed cylindrical shell with means near the top for the introduction of fragmented coal and also near the top is an offtake through which gas produced from the coal is conducted from the interior of the shell. There is a grate structure within the shell located above the bottom of the shell providing an ash-pit below the grate as well as provision for the introduction of combustion air up through the grate for burning char, which is the residual coal material from which volatiles, gas and vapors, have been stripped in the operation of the gasifier. The burning of the char, which at this stage has become similar to coke, provides heat and hot gases for retorting the bed or column of coal above the fire zone. The grate is in the form of an eccentric stepped cone, the bottom tier of which extends across the entire diameter of the shell with several progressively smaller tiers terminating in a central, conical tier. The grate supports the entire burden of the charge within the cylindrical shell, starting with a bed of ash. Above the bed of ash is the fire zone, in which there is an exothermic oxidation of the char, and above the fire zone there is a reduction zone in which the gases produced by oxidation in the fire zone are reduced in an endothermic reaction. Above the reduction zone there is a depth of fragmented coal from which gas and volatiles are being distilled. Assuming the gasifier to be in operation, sufficient air is supplied through the grate and the overlying bed of ash to support combustion in a layer of devolatilized coal, or char, above the grate. The air supplies oxygen in a controlled volume to support fire in the fire zone only and without oxygen combustion cannot occur above this level. The heat of combustion and hot combustion gases rise through the bed of coal, and the coal immediately above the fire zone, in the reduction zone, is subjected to a chemical reaction which reduces the oxygen content of the gases and vapors entering this zone from the fire zone. The coal above the reduction zone becomes hot enough to release the volatiles, mostly gas and some vapors, which with combustion gases pass higher up through the bed to preheat the coal above, the gas, volatiles and spent combustion products then being withdrawn through the upper end of the gasifier for use as fuel.
As previously explained, distillation of the volatile products from the coal just above the fire zone converts the coal, stripped of its volatiles, into char. As the char in the fire zone burns, it produces ash and gases, and the newly formed char above the fire zone moves down to replace the char that has been oxidized and this cycle continues as long as the coal in the gasifier is replaced.
It is critical to this process that the fire zone and the ash bed be maintained at a fairly constant depth, which means that the air flow up through the grate must be uniformly distributed. If the depth of the ash bed diminishes to the extent that the hot char contacts the grate surface, warping, distortion, or other physical damage may occur. Conversely, if the depth of the ash bed should increase to the extent that the remaining available depth of coal is insufficient to permit the gasification reactions to occur, the resultant gas quality will diminish. If the air flow through some areas of the bed becomes clogged, the air will "channel" into the unclogged areas of least resistance and the gasification reactions will deteriorate.
It is for this reason that an attendant must be able to poke into and around the fire zone, primarily to periodically measure the depth of the ash bed, and based on this measurement, adjust the speed of the grate rotation to keep this depth constant, and occasionally, also to break up agglomerated masses and clinkers. To effectively accomplish this there must be "pokeholes" around the shell at spaced intervals. The pokeholes must be closed when they are not in use, and each has its individual closure, such as a hinged door, which is opened only when the attendant wants to use the poker in that sector of the bed to which the door and pokehole give access.
Since the pressure inside the gasifier is necessarily slightly higher than atmospheric pressure outside of it, the opening of a pokehole door is always accompanied with an outthrust of hot gases or vapors and usually dust, making the work of the attendant both disagreeable and hazardous. Pokehole doors are commonly located at the top of the gasifier vessel, or on the sides of the vessel above the reduction zone. In some cases, means are provided to allow the admission of inert gas or steam into the pokehole to prevent the outthrust of gases and dust. The introduction of such inert gases, however, results in a dilution of the gas stream flowing from the offtake from the vessel.